A conventional telecommunications private network comprises a number of private exchanges or switches (PBX's) interconnected via private telephone lines and each of which serves a number of telephone or user terminals. The PBX's of the private system are interconnected by private leased lines which are installed by the appropriate telephone service supplier. Call routing and call handling within the private network are controlled via the PBX's.
In addition to a basic telephony service (POTS) a private network is generally required to provide additional features such as call forwarding, call transfer, ring-back when free and conference calls. These features are not in general provided on a public network as they are specific requirements of business rather than domestic subscribers.
A further service that is finding increasing usage is that of providing a direct dialling facility to telephone extensions attached to a PBX. For example the CENTREX system provides such a service.
An approach to the problem of integrating public and private networks is described in specification No. WO 93/15583 which relates to a method of providing system and network independent dedicated private circuit telecommunications functionality using separate signalling and transmission network bearers.
The provision of private leased lines represents a significant capital investment by the telephone service supplier. Furthermore these leased lines represent an underused asset as they are in significant use, for only a part, typically about one third, of the twenty four hour cycle. However, during their idle period, these lines are not available to carry telephone traffic, e.g. to reduce an overload, for subscribers other than the private user to whom the lines have been leased. Expansion of the network is also difficult, as new dedicated lines have to be provided for new business subscribers. Various workers have addressed this problem by providing systems in which a private network comprises a number of local networks interconnected by non-dedicated public network paths. This however introduces the problem that certain features provided on the private network may not be available on the public network and there is thus a need to provide feature transparency for private network calls routed across the public network, i.e. to provide private network users with the perception that their network comprises a single seamless network.